The black hole image confirms those who have seen the invisible



Black holes are the undisputed titans of the cosmos. These collapsed stars are so dense and massive that even light can not escape their gravitational claws, making them invisible.

Do not be afraid: we can now "see the invisible", according to astronomers who unveiled Wednesday the first picture of the black hole and its surroundings. Admittedly, it looks like a donut: a circle of darkness surrounded by a fuzzy and brilliant light. The image of the supermassive black hole, which is 6 billion times more massive than our sun and located 53 meters-light from the Messier 87 galaxy, was captured in fragments by a global network of high-altitude terrestrial telescopes. then reconstituted.

While the confirmation burns the legacy of Albert Einstein – it confirms his general theory of relativity, even though he did not believe such objects existed – it should also inspire admiration of John Michell, an English rector of the 18th century who theorized the existence of "black stars" before the birth of Einstein. Michell is the first often forgotten link in a chain of scientists who have applied their extraordinary mind to the bizarre implications of these extreme objects. So extreme that, for example, a person falling in a black hole would suffer a "spaghettification" or an extraordinary stretch due to the huge gravitational pull.

Michell, born in 1724 and educated at the University of Cambridge, was described by his contemporaries as being small and fat. Fortunately, the rector of Thornhill, near Leeds, was also "regarded as a very ingenious man and an excellent philosopher." Among his friends were Joseph Priestley, a scientific clergy member who was behind the discovery of oxygen, and Henry Cavendish, who discovered hydrogen. A passionate astronomer, Michell first turned to Isaac Newton 's gravity equations to see if they could be used to deduce the masses of stars.

He grasped the concept of leak velocity, the speed needed by an object to escape a gravitational field (calculated regularly for rocket launches). In 1783, Michell wrote a letter to Cavendish suggesting that there might be such massive "black stars" that "all the light emitted by such a body would be brought back to him by his own gravity." They would be invisible.

In addition to Pierre-Simon Laplace, who independently suggested a similar concept in 1796, the idea of ​​black stars – or black holes, as they were renamed in the 1960s – remained latent until Einstein proposed his general theory of Relativity, published in 1916. His equations contained the possibility of "singularities" infinitely dense and light. While Einstein ignored them, Karl Schwarzschild was intrigued. The same year, the German astronomer calculated that once the star collapsed within a certain radius, the space-time would collapse. He died shortly after.

By the way, in 1930, Subrahmanyan Chandrasekhar, aged 19, interrupted his India trip to England speculating on the internal forces acting on a star run out of nuclear fuel. Would it contract by gravity to a certain point and then stop? Chandrasekhar supposed that stars whose mass was at least 1.4 times that of our sun would collapse indefinitely.

Einstein, still troubled, published an article in 1939 suggesting that Schwarzschild had miscalculated. The war then follows: the American physicist Robert Oppenheimer, who uses Einstein's theory to show that such singularities can form, is diverted from his work on black holes to develop the atomic bomb.

In the 1960s, John Wheeler revived interest in Einstein's work and popularized the term "black hole" for these singularities, captivating a new generation of researchers. The late British scientist Stephen Hawking was one of them: he calculated that black holes should evaporate through a process called "Hawking radiation". The "Schwarzschild radius" and the "Chandrasekhar limit" have also entered the black hole nomenclature.

More recently, astronomers have amassed indirect evidence of black holes throughout the cosmos, through their effects on nearby stars. We now know that a supermassive black hole is at the heart of our own galaxy, the Milky Way. This is the next target of the Horizon telescope of the event.

Now, however, we have our first direct confirmation of a "black star" – a revelation that would have delighted Michell and possibly exasperated Einstein.

The author is a scientific commentator


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